20 DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. whilst that of one of the higher animals resides in the sum of all its organs. 3. The very simplest Organized fabric is further distinguished from Inorganic bodies by marked differences in regard to intimate structure and consistence. Inorganic substances can scarcely be regarded as possessing a structure, since their perfection consists in their homogeneousness and their solidity. It is the essential character of Organized fabrics, on the other hand, that they are formed by a combination of solid and liquid components, so intimately combined and arranged as to impart a heterogeneous character to almost every portion of their substance; and in all the parts which are most actively concerned in the vital operations, softness of texture seems an essential condition,those parts only being so consolidated as to acquire anything comparable to the density of mineral bodies, which are destined to possess the simply physical property of resistance, so as to be subservient either to support, to protection, or to mechanical movement. A comparison between the pulpy portion of the leaves of Plants and the heartwood of the stem, between the membranous tissues of the Coral-polypes and the stony masses which they form, between the firm shell of the Crab or the Oyster and the substance of the included body, or between the solid bones of Man and the flesh which clothes them, will serve to illustrate this principle. It is in such solidified portions of the Organized fabric, that the greatest resemblance exists to Inorganic bodies; but even these portions all pass through the condition of soft tissue, the consolidation of which is effected by the deposit of some hardening material (generally carbonate or phosphate of lime), in its interstices.-It is by the reaction which is continually taking place between the solid and the liquid parts of Organized structures, that their integrity is maintained. For we shall find it to be a result of their peculiar composition, that they are prone to continual decay; and this decay would speedily destroy them altogether, if it were not compensated by new formation. The materials for their reproduction must always be presented to the tissues in a liquid state, and all the dead and decomposing matter must be reduced to the same form, in order that it may be carried off; so that the intermingling or mutual penetration of solids and liquids, in DISTINCTIVE CHARACTERS OF ORGANIZED BODIES. 21 the minutest parts of Organized bodies, is a necessary condition of their existence. 4. Organized structures are further distinguished from Inorganic masses by the peculiarity of their chemical constitution. This peculiarity does not consist, however, in the presence of any elementary substances which are not found elsewhere; for all the elements of which Organized bodies are composed, exist abundantly in the world around. This, indeed, is a necessary consequence of the mode in which they are built up; for that which the parent communicates in giving origin to a new being, is not the structure itself, but the capacity to form that structure from materials supplied to it; and it is by progressively converting these materials to its own use, that the germ develops itself into the complete fabric.-Now out of about seventy simple or elementary substances which are known to occur in the Mineral world, not above twenty present themselves as constituents of Vegetable and Animal fabrics; and many of these occur there in extremely minute proportion. Some of them, indeed, appear to be introduced merely to answer certain chemical or mechanical purposes; and the composition of the parts which possess the highest vital endowments is extremely uniform. They are nearly all formed at the expense of certain "organic compounds," which are made up of the four elementary substances, oxygen, hydrogen, carbon, and nitrogen; and these elements appear to be united, not as in the case of inorganic compounds, two by two, or after the binary method, but all four together, so as to form a compound atom of great complexity. Thus common nitre is regarded as a binary compound of nitric acid and potass, since it can be decomposed into those two constituents and can be re-formed by their union; and in the same manner, its nitric acid is a binary compound of nitrogen and oxygen, whilst its potass is a binary compound of potassium and oxygen. But neither albumen nor gelatine, which are the principal materials of the animal tissues, can be resolved into any two other substances, by the union of which it can be re-formed; and when once it has been decomposed by chemical agencies, no means known to the chemist can reproduce it. Albumen can, in fact, be generated only by the living Plant, at the expense of the carbon, hydrogen, oxygen, and nitrogen, which it draws from the elements around; and 22 DISTINCTIVE CHARACTERS OF LIVING ORGANISMS. gelatine can only be formed in the animal body by a metamorphosis of the albumen which it derives from the Plant. The peculiar mode in which the elements of these substances are held together, renders them very prone to decomposition; so that Organized bodies, when no longer alive, rapidly pass into decay, unless they are secluded from the contact of oxygen, or are kept at a very low temperature. Such decay, however, is continually taking place during life, and would make itself obvious if its products were not carried out of the system as fast as they are generated within it. It essentially consists in the resolution of the four principal components of organic compounds-carbon, hydrogen, oxygen, and nitrogen, in combination with oxygen drawn from the atmosphere-into the three binary compounds, water, carbonic acid, and ammonia, which thus restore to the Inorganic world the original materials of Organized fabrics, in the very forms from which those materials were first derived by the agency of the growing Plant. (See VEGET. PHYSIOL.) There 5. It is, however, by their peculiar actions, that living Organisms are most completely differentiated from the inert bodies of which the Mineral kingdom is composed. can be no doubt that of many of the changes which take place during the life of an Organized being, a large proportion (especially in the Animal kingdom) are effected by the direct agency of physical and chemical forces; and there is no reason to believe that these forces have any other operation in the living body, than they would have out of it under similar circumstances. Thus the propulsion of the blood by the heart, through the large vessels, is a purely mechanical phenomenon; as is also the movement of the limbs by the lever-action of the forces brought to bear on their bones. So, again, the digestive operations which take place in the stomach are of a purely chemical nature; and the interchange of gases between the air and the blood, which takes place in the act of respiration, must be regarded in the same light.But after every possible allowance has been made for the operation of physical and chemical forces in the living organism, there still remain a large number of phenomena which cannot be in the least explained by them, and which must be regarded as the result of an agency that differs from these as they differ from each other; and this agency, which DISTINCTIVE CHARACTERS OF LIVING ORGANISMS. 23 is recognised by the effects it produces-in the same manner as we recognise heat or electricity by their effects-may be conveniently designated vital force.1 Thus, to revert to our previous illustrations, the mechanical power employed in the propulsion of the blood, or in the movements of the limbs, is evolved by muscular contraction, a phenomenon altogether peculiar to the living muscle; and the muscle derives its property of contractility from the previous development of its peculiar tissue in the act of nutrition. So the solvent fluids by which the digestion of food is accomplished, aro separated from the blood by an act of secretion, which can only be performed by a glandular apparatus in the living walls of the alimentary canal. And the materials for the nutrition of the muscular tissue, and for the secretion of the digestive solvent, as of all the other acts of nutrition and secretion which are continually going on in the living body, are derived from the blood,-a liquid which possesses properties very different (as we shall hereafter see) from any mere mixture of chemical compounds, and which is prepared by actions totally beyond the power of the chemist to imitate,the laboratory of the living organism being requisite for their performance. 6. The whole assemblage of vital actions which is performed by the living Animal, may be arranged under two principal groups; one of them consisting of those which are directly concerned in the development and maintenance of its Organized fabric; the other including all those by which it is brought into conscious relation with the world around. The former group includes the acts of digestion, absorption, and assimilation, by which the nutritive materials are prepared for becoming part of the living fabric; the circulation of the assimilated materials through the body; their conversion, by the act of nutrition, into the solid textures; the formation of various secretions, having various purposes to serve in the economy; the removal, by the acts of respiration The Author has elsewhere given his reasons for the belief, that Vital force bears the same "correlation" to the Physical and Chemical forces, as the latter bear to each other; but the discussion of this subject is not suited to an elementary treatise; and the essential peculiarity of the manifestations of vital force in the phenomena of life, requires that it should be treated as belonging to a distinct category. 24 DISTINCTIVE CHARACTERS OF ANIMALS. and excretion, of the effete matters with which the blood becomes charged by the decomposition continually going on in the body; the maintenance of animal-heat by the same process; and the act of reproduction, whereby the race is perpetuated, in spite of the limited duration of the individual. The foregoing, which are for the most part common to the Animal and the Plant, are termed Organic Functions, or Functions of Vegetative Life. But, in addition to these, it is the characteristic of Animals generally, that they are sensible to impressions made by surrounding objects, so that they possess some consciousness of what is going on about them; and that they also possess the power of re-acting on those objects by movements of their own, so as to change either their own places, or the places of surrounding objects in relation to themselves. These two functions, sensibility and the power of spontaneous motion, being peculiar to animals, are distinguished as Animal Functions, or Functions of Animal Life. In the higher animals, they are the most important and characteristic phenomena of their existence; so that it would seem as if the whole assemblage of organic functions had no other destination in them, than to build up and keep in order the apparatus by which the functions of animal life are performed. But this state of things is entirely reversed among those lower tribes of animals which border most closely on the Vegetable kingdom; for we find that among such, the manifestations of sensibility and power of spontaneous movement are so feeble, that it may be doubted whether these attributes are really present in them; and even in higher orders, there are many in which the proper animal powers are in such a low grade of development, that they appear as if they were destined merely to minister to the organic functions. 7. Thus, although the characteristic difference between the Animal and the Vegetable kingdom, taking each as a whole, may be truly said to consist in the possession by the former of endowments which do not exist in the latter, this does not express the essential difference between Animals and Plants; since, while there are many tribes among the former in which the proper animal powers are reduced to so low a degree as to prevent it from being certainly affirmed that they are present at all, there are many tribes among the lower plants which exhibit a power of spontaneous movement fully as |